Draft: metadata: Save context managers for opening files

ruamel.yaml also accepts pathlib.Path objects; no need to open a YAML file ourselves.

Ref.: https://yaml.readthedocs.io/en/latest/basicuse.html

This feature has been added in version 0.15.0 of ruamel.yaml which is the minimum version supported by fdroidserver already.

Ref.: https://sourceforge.net/p/ruamel-yaml/code/ci/default/tree/CHANGES